To compare two commercial lighting layouts, put the assumptions behind each one side by side before you look at fixture counts. Check four groups: the room and its zones, the tasks and stated criteria, the mounting conditions, and the operating and control assumptions.
Mark each group as aligned, different, or missing. If an assumption that affects the result differs or is missing, ask for clarification or a revised layout using the same stated basis. Fixture counts are useful only when both proposals address the same problem.
Compare the Proposals on a Common Basis
First confirm that both proposals refer to the same project and drawing revision. Then record what each layout states for each assumption group. Note any gaps and ask the bidder to clarify them.
| Assumption | Compare in both layouts | If different or missing |
|---|---|---|
| Room and use | Boundaries, dimensions, included zones | Ask which area each covers |
| Tasks and criteria | Task per zone, stated targets, calculation basis | Request a shared stated basis |
| Mounting and constraints | Locations, height, obstructions | Request revised placement or a note |
| Operation and controls | Control zones, schedules, modes | Ask whether both should use one basis |
Give each row a status for both proposals. Aligned means both layouts state the same input. Different means both state it, but the values do not match. Missing means one or both proposals leave it out.
Review fixture count last. Treat it as one data point alongside the stated results. For example, Proposal A might use fewer fixtures because it leaves out a loading zone or assumes a lower mounting height. Then the count difference reflects different problems, not better design. Once every row is aligned, differences in count reflect actual layout choices that you can weigh against the stated results.
Confirm Both Layouts Cover the Same Room and Zones
The room check passes only when both proposals describe the same boundaries, dimensions, and zones. Check each plan for these items:
- Drawing reference: the project name, area, and revision date or number on each layout.
- Room boundary: where each plan starts and stops, including walls, mezzanines, or adjacent spaces.
- Dimensions: the length, width, and ceiling or clear height used in the calculation.
- Included zones: which areas are lit, such as aisles, work areas, staging, or offices.
A scope gap is not a layout difference. If one proposal includes the shipping dock and the other does not, the fixture totals will differ for reasons unrelated to design quality. If the scope differs or is unclear, ask the bidder for a corrected drawing or a written note naming the intended area. Then compare the proposals again using the same scope.
Compare the Intended Tasks and Reported Criteria
Two layouts are comparable only when they target the same task in each zone and report results on a comparable basis. Compare proposals zone by zone, not just by whole-room totals. Record the intended use, target criteria, and photometric calculations provided for each zone.
If one proposal treats an area as bulk storage and the other treats it as an inspection area, they are answering different questions. Our storage-versus-inspection example shows how the use of a shared space can affect the layout decision.
Calculation inputs matter as much as the reported numbers. VA projects provide a concrete example of a documented basis. For those projects, the VA calculation assumptions and reported results call for indoor calculation grids no larger than 2 ft × 2 ft at 2 ft 6 in above finished floor, unless otherwise noted. They also call for surface reflectances that match the specified room finishes and appropriate light loss factors. A light loss factor accounts for light output lost over time. The same VA manual asks reports to cover all functional areas and list average, maximum, and minimum illumination; luminaire types and quantities; light source types; and the light loss factor used.
Those provisions apply to VA work, not every US commercial project. Still, they show why you should review fixture counts alongside the results. If proposals use different grids, reflectances, or loss factors—or leave them out—their reported light levels are not directly comparable. Ask both bidders to state those inputs on the same basis, and use the project's stated criteria when reviewing the results. Our guide to specific IES files explains how luminaire data can change a calculation. If proposals cite target light levels by zone, our guide to foot-candle levels by warehouse zone explains where those targets come from.
Check Mounting Assumptions Against Physical Constraints
Matching fixture counts can hide very different placement assumptions. Compare what each proposal says about mounting against the project's documented conditions, especially in retrofits where existing positions or obstructions limit fixture placement. For each layout, record:
- Mounting locations: whether fixtures sit at existing points or new positions.
- Mounting height: the height used in the layout and whether it matches the documented ceiling or structure.
- Obstructions and fixed conditions: racks, ducts, beams, cranes, or equipment shown in the project information.
- Stated differences: any place where a proposal's mounting basis differs from the documented conditions.
If placement changes, ask for a revised layout showing the changed assumption. Ask the bidder to clarify any missing or conflicting mounting detail against the documented conditions. A project professional should confirm whether installation is suitable. For background on how ceiling types shape that conversation, see our guide to high bay mounts by ceiling type.
Compare Operating Assumptions and Control Zones
Compare how each layout expects the space to operate. Two proposals may be based on different conditions of use. For each proposal, record:
- Control zones: which fixtures are grouped and controlled together.
- Schedules: stated operating hours or time-based control periods, if included.
- Operating modes: any stated occupancy, daylight, or partial-output conditions.
- Gaps: where one proposal states an operating basis and the other does not.
If only one proposal includes a schedule, or the proposals describe different operations, ask whether both should use the same basis. When an operating assumption is missing, ask the bidder to state it rather than filling in a schedule yourself. On VA projects, the VA lighting design manual requires designers to define automated lighting control zones and include a Lighting Control Zone Schedule on the drawings. That requirement applies to VA projects. For other jobs, it shows the kind of control detail you can ask both bidders to provide in writing.
If neither proposal states its basis clearly, we can provide a lighting design layout using the same room, task, mounting, and operating inputs you documented. This gives you a reference built on a known basis.
FAQs
What does it mean if two layouts have the same fixture count?
Not much on its own. Two proposals can have the same count while covering different zones, assuming different mounting heights, or using different calculation inputs. A matching count suggests similar outcomes only when the assumptions behind both layouts align.
What should I ask for when a proposal leaves out its assumptions?
Ask for the missing inputs in writing: the area and zones covered, the task and criteria for each zone, the mounting basis, and the control or operating basis. Ask each bidder for the same items so you can compare their answers.
Can I apply the VA manual's calculation grid to a private commercial project?
The VA manual governs VA projects, so its grid is not a requirement for other work. You can still ask both bidders to state the grid spacing and height they used. A shared, stated grid makes their reported light levels easier to compare.
Does aligning the assumptions mean a layout meets code?
No. Aligning assumptions makes the proposals comparable. It does not confirm design adequacy, energy code compliance, or electrical suitability. Those decisions belong to the lighting designer, engineer, electrician, or local authority having jurisdiction for the project's location.
